The purpose of this proposal is to develop a modification of Teller's forced-choice preferential looking (FPL) procedure, to allow clinical assessment of visual acuity in human infants. Four types of studies are in progress: 1. Clinically related studies conducted in the laboratory. A laboratory procedure for measurement of acuity of children between 5 months and 3 years of age has been developed. The procedure is called operant preferential looking (OPL). A modification of the OPL procedure that will allow quick, efficient screening of acuity of children seen clinically will be developed. An improved means of occluding one eye for monocular testing is being developed. This occluder will be used for monocular testing of young infants with the FPL procedure and older infants and toddlers with the OPL procedure. 2. Testing of infants in clinical settings. The clinical usefulness of the procedure and the applicability of preliminary laboratory norms to patient populations are being assessed through tests of infants seen at local well-baby and high-risk infant clinics and through assessment of a group of infants with Down's syndrome. 3. Basic laboratory studies. No systematic variation in acuity as a function of luminance was found for luminance values above -0.3 log cd/m2. Tests of the effects of variation in luminance below -0.3 log cd/m2 are in progress. Initial results of a study of the effects of defocus on infant acuity suggest that infants are less sensitive than adults to optical blur. 4. Apparatus development. Construction of an apparatus for photorefraction is planned. This will allow refraction of all infants who participate in acuity testing.